The Microbial Communities of the Future

The world, it turns out, is getting warmer. The extent and precise contours of climate change may never be forecastable, but many scientists are working to test the repercussions of a warmer atmosphere, in order to both inform policy discussions and instigate difficult discussions about adaptation strategies.

Nicholas Bouskill, an ecologist at the University of California at Berkeley, is one such scientist. Through a carefully designed study of jungle floor patches in Puerto Rico, Bouskill and his team are hoping to determine how the microbial make-up of soil changes in response to repeated periods of dryness. “These locations are likely to experience changes in the magnitude of rainfall, with increased drought and longer dry periods,” he writes in a recent edition of the ISME Journal.

It wasn’t necessarily clear how a drier environment would impact the microbial community since soil moisture is a double edged sword: Too much of it, and the diffusion of important gases in and out of the soil is limited; too little, and nutrients might not reach the microbes in sufficient quantities. So how, and how quickly, might soil-based microbial communities change in response to less water? Bouskill sought answers by tracking diversity shifts in plots of soil cut off from rain throughfall for the first time and those experiencing a second artificial drought.

Here’s how the experiment went down. In June of 2008, the scientists set up corrugated plastic canopies over five patches of soil for three months. The next year, they returned, erecting canopies over the same five squares as well as five new ones. Unaltered areas were also included in the study as an experimental control.

With these three experimental conditions, the team would be able to tell not only how less direct water would affect the microbial community, but also how “priming” the soil with drought a year earlier affected things.

The soil’s water chemistry changed significantly, as rainfall exclusion led to higher sodium concentrations, while iron, aluminum, and phosphorous levels were lower. Shifts in the soils’ microbial community compositions, however, were more difficult to predict. Remarkably, Bouskill and his team found that the lower rainfall levels had no discernible effect on the overall amount of soil biomass. The covered patches were just as full of microbial life as the uncovered plots, but they were more homogenous, as genetic diversity dove by 40 percent. Soils experiencing a second artificial drought saw diversity levels bounce back to those of the control plots. As Bouskill puts it, the effect of repeated water exclusion “manifested as changes in the relative abundance of organisms, but not as decreased diversity.”

So how might the diversity of soil patches in Puerto Rico affect our predictions for microbial life in a warming world? Beyond the specific prescriptions for tropical rain forests (more Planctomycetes and Actinobacteria), it appears that repeated exposure to a new climate regime bolsters a community’s ability to maintain diversity. The relative abundances of particular species may change drastically, but the overall amount of genetic diversity appears to remain relatively constant.

This is an encouraging result for the planet’s biodiversity, but the jury’s still out on how ecosystems will change in other respects. For example, how will overall metabolic cycling of nutrients like carbon or nitrogen change? And how would such changes affect the rest of the jungle? After all, the soil’s microbial soup is the unseen arbiter of the jungle’s health, and any substantial changes in microbial composition, diversity, or abundance will change the entire system in unpredictable ways. Hopefully, the surprisingly robust microbial diversity will help curtail drastic ecosystem-wide changes.